IANA Timezone Database Explained

The IANA Timezone Database

The IANA timezone database, also known as the tz database, tzdata, or the Olson database (after its original creator Arthur David Olson), is the most widely used source of timezone information in the world. It is maintained by a community of volunteers under the coordination of ICANN (Internet Corporation for Assigned Names and Numbers).

Structure and Naming Conventions

Timezone identifiers follow the pattern Area/Location, where:

• Area is a continent or ocean (e.g., America, Europe, Asia, Pacific)
• Location is typically the largest city in the timezone region (e.g., New_York, London, Tokyo)

America/New_York       # Eastern US
Europe/London          # United Kingdom
Asia/Tokyo             # Japan
Pacific/Auckland       # New Zealand

Some timezones have three-part identifiers for sub-regions:

America/Argentina/Buenos_Aires
America/Indiana/Indianapolis
America/North_Dakota/Center

Why Not Use Offsets or Abbreviations?

A timezone ID encodes the complete history of offset changes, including when daylight saving time was adopted or abolished, political boundary changes, and historical adjustments. An offset like UTC-05:00 only tells you the current difference from UTC, not whether DST applies or when it transitions.

Abbreviations like EST (Eastern Standard Time) are ambiguous — CST could mean Central Standard Time (US), China Standard Time, or Cuba Standard Time.

How It Is Updated

The database is updated several times per year (typically 5-10 releases) as governments change their timezone rules. Each release is named with the year and a letter, such as 2024a, 2024b, etc. Updates propagate to operating systems, browsers, programming languages, and databases through their respective update mechanisms.

Data Contents

Each timezone entry contains:

1. UTC offset for standard time
2. DST rules (if applicable) including transition dates
3. Historical changes going back to the adoption of standard time
4. Leap second information (in a separate file)